Anti-torque systems for rotorcraft

ABSTRACT

An anti-torque system for a rotorcraft includes a first tail fan assembly including a plurality of first fan blades and a second tail fan assembly including a plurality of second fan blades. The first tail fan assembly has a larger diameter than the second tail fan assembly. The first fan blades have a larger rotational inertia than the second fan blades such that the second fan blades experience a larger angular acceleration than the first fan blades in response to torque, thereby providing yaw control for the rotorcraft.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. application Ser. No.15/896,307 filed Feb. 14, 2018 and is a continuation-in-part of U.S.application Ser. No. 16/886,000 filed May 28, 2020, which is adivisional of U.S. application Ser. No. 15/458,525 filed Mar. 14, 2017,now U.S. Pat. No. 10,703,471, which is a continuation-in-part ofapplication Ser. No. 15/172,811 filed Jun. 3, 2016, now U.S. Pat. No.10,377,479.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to anti-torque systemsoperable for use on rotorcraft and, in particular, to anti-torquesystems including two or more tail fans having different rotationalinertias to meet the thrust, responsiveness and other requirements ofthe rotorcraft.

BACKGROUND

The main rotor of a helicopter, which produces lift necessary forflight, also produces a counteracting torque force on the fuselage ofthe helicopter, turning the tailboom of the helicopter in the oppositedirection of the main rotor. The helicopter's tail fan, or rotor,located aft of the main rotor, is used to counteract this torque andcontrol the yaw of the helicopter. One consideration regarding theperformance of a tail fan is the tail fan's capability to outputsufficient anti-torque thrust to maintain control of the yaw of thehelicopter. Typically, the tail fan is required to output a higheranti-torque thrust when the helicopter is performing a sideward flightmaneuver or experiencing strong side winds. In such circumstances, thetail fan may be required to output anti-torque thrust at or near itsmaximum capabilities. Another consideration regarding the performance ofthe tail fan is its responsiveness when changing or correcting the yawof the helicopter. Ideally, a tail fan should be able to change speedrapidly to quickly and precisely control the helicopter's yaw. Currenttail fans, including both fixed and variable rotational speed systems,face difficulties in meeting both the thrust and responsivenessrequirements mentioned above. For example, while a large tail fan may beable to easily meet the maximum anti-torque thrust requirement formaintaining yaw in sideward flight conditions, the correspondingly largerotational inertia of the tail fan may hinder its responsiveness whenquick yaw adjustments are necessary. Conversely, a smaller tail fanhaving a lower rotational inertia can more easily meet responsivenessrequirements, but may be unable to output the anti-torque thrustrequired by the helicopter in all operational circumstances.Accordingly, the need has arisen for an improved anti-torque system thatis capable of meeting the thrust, responsiveness, cost and otherperformance requirements of rotorcraft.

SUMMARY

In a first aspect, the present disclosure is directed to an anti-torquesystem for a rotorcraft including a first tail fan assembly including aplurality of first fan blades and a second tail fan assembly including aplurality of second fan blades. The first tail fan assembly has a largerdiameter than the second tail fan assembly. The first fan blades have alarger rotational inertia than the second fan blades such that thesecond fan blades experience a larger angular acceleration than thefirst fan blades in response to torque, thereby providing yaw controlfor the rotorcraft.

In some embodiments, the first and second fan assemblies may be fixedpitch, variable rotational speed fan assemblies. In other embodiments,the first and second fan assemblies may be variable pitch, fixedrotational speed fan assemblies. In certain embodiments, the first tailfan assembly may form a larger rotor disk diameter than the second tailfan assembly. In some embodiments, the first fan blades may be longerthan the second fan blades. In certain embodiments, the first fan bladesmay be formed from a different material than the second fan blades. Insome embodiments, the plurality of first fan blades may include a largernumber of fan blades than the plurality of second fan blades. In certainembodiments, the first fan blades may be wider than the second fanblades.

In some embodiments, the first fan blades may include a circumferentialtip ring. In certain embodiments, at least one of the first or secondtail fan assemblies may be a shrouded tail fan assembly. In someembodiments, at least one of the first or second tail fan assemblies maybe an open tail fan assembly. In certain embodiments, the first tail fanassembly may include a plurality of first tail fan assemblies and thesecond tail fan assembly may include a plurality of second tail fanassemblies. In some embodiments, the anti-torque system may include atleast one motor configured to provide torque to the first and second fanblades. In certain embodiments, the at least one motor may include firstand second motors. In such embodiments, the first tail fan assembly mayinclude the first motor and the second tail fan assembly may include thesecond motor. In some embodiments, the motor may be an electric orhydraulic motor. In certain embodiments, the motor may be a variablespeed motor. In some embodiments, the first fan blades and the secondfan blades may be formed from the same material.

In a second aspect, the present disclosure is directed to a rotorcraftincluding a fuselage, a tailboom extending from the fuselage and ananti-torque system at least partially located at the aft portion of thetailboom. The anti-torque system includes a first tail fan assemblyincluding a plurality of first fan blades and a second tail fan assemblyincluding a plurality of second fan blades. The first tail fan assemblyhas a larger diameter than the second tail fan assembly. The first fanblades have a larger rotational inertia than the second fan blades suchthat the second fan blades experience a larger angular acceleration thanthe first fan blades in response to torque, thereby providing yawcontrol for the rotorcraft.

In some embodiments, the aft portion of the tailboom may include avertical fin, and the first and second tail fan assemblies may becoupled to the vertical fin. In some embodiments, the rotorcraft mayinclude a flight control computer including an anti-torque controller incommunication with the first and second tail fan assemblies. In suchembodiments, the anti-torque controller may be operable to control theyaw of the rotorcraft using the first and second tail fan assemblies. Incertain embodiments, the anti-torque controller may include a yaw changedetermination module operable to determine a yaw adjustment for therotorcraft and a tail fan control module operable to modify the yaw ofthe rotorcraft using the first and second tail fan assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1D are schematic illustrations of a rotorcraft including ananti-torque system in accordance with embodiments of the presentdisclosure;

FIGS. 2A-2B are side views of anti-torque systems for rotorcraft inaccordance with embodiments of the present disclosure; and

FIGS. 3A-3B are side views of anti-torque systems for rotorcraft inaccordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,all features of an actual implementation may not be described in thisspecification. It will of course be appreciated that in the developmentof any such actual embodiment, numerous implementation-specificdecisions must be made to achieve the developer's specific goals, suchas compliance with system-related and business-related constraints,which will vary from one implementation to another. Moreover, it will beappreciated that such a development effort might be complex andtime-consuming but would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicesdescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including by mere contact or by moving and/or non-movingmechanical connections.

Referring to FIGS. 1A-1D in the drawings, a rotorcraft is schematicallyillustrated and generally designated 10. In the illustrated embodiment,rotorcraft 10 is depicted as a helicopter having a main rotor 12, whichincludes a plurality of rotor blade assemblies 14. Main rotor 12 isrotatable relative to a fuselage 16. The pitch of rotor blade assemblies14 can be collectively and/or cyclically manipulated to selectivelycontrol direction, thrust and lift of rotorcraft 10. A landing gearsystem 18 provides ground support for rotorcraft 10. A tailboom 20extends from fuselage 16. Main rotor 12 rotates in a direction indicatedby arrow 22, which produces a torque on fuselage 16 in a directionindicated by arrow 24. Aft portion 26 of tailboom 20 includes ananti-torque system 28, which is operable to counteract torque 24 andgenerally control the yaw of rotorcraft 10.

Helicopters have traditionally included a single tail fan, or rotor,located at aft portion 26 of tailboom 20 to counteract torque 24. Asused herein, including in the claims, a fan includes both fixed pitch,variable rotational speed rotor systems and/or variable pitch, fixedrotational speed rotor systems. A single tail fan, however, compromisesthe maximum thrust and responsiveness requirements of the helicopter. Inparticular, a large tail fan may be sufficient to counteract torque 24in all operational circumstances, but may be unable to adjust or correctthe yaw of rotorcraft 10 in a sufficiently responsive manner by virtueof the tail fan's high rotational inertia. Conversely, a small tail fanwith a low rotational inertia may be sufficiently responsive, but unableto meet the anti-torque thrust demands necessary to counteract torque 24in some operational circumstances, such as during sideward flight orsideward wind conditions. To address these and other deficiencies ofcurrent helicopters, anti-torque system 28 includes two tail fanassemblies 30, 32 located at aft portion 26 of tailboom 20. In theillustrated embodiment, aft portion 26 of tailboom 20 also includes avertical fin 34 to which tail fan assemblies 30, 32 are rotatablycoupled. Tail fan assembly 30 includes a motor 36 that provides torqueto fan blades 38. Similarly, tail fan assembly 32 includes motor 40 thatprovides torque to fan blades 42. Fan blades 38, 42 may be fixed pitchfan blades and motors 36, 40 may be variable speed motors capable ofproducing a wide range of revolutions per minute (RPM).

Fan blades 38 of tail fan assembly 30 have a larger rotational inertia,or moment of inertia or angular mass, than fan blades 42 of tail fanassembly 32. In the illustrated embodiment, fan blades 38 have a largerrotational inertia by virtue of being longer than fan blades 42. Fanblades 38 thus form a larger rotor disk, in terms of area and diameter,than fan blades 42. Because fan blades 42 are shorter than fan blades38, fan blades 42 are adapted to experience a larger angularacceleration in response to torque from motor 40 than that experiencedby fan blades 38 in response to torque from motor 36. On the other hand,fan blades 38 are capable of producing a larger maximum anti-torquethrust 44 than the maximum anti-torque thrust 46 produced by fan blades42. In this manner, anti-torque system 28 provides responsive yawcontrol for rotorcraft 10 using tail fan assembly 32 while remainingcapable of providing the anti-torque thrust required in all operationalcircumstances, including sideward flight or side wind conditions, usingtail fan assembly 30. Tail fan assembly 30 is capable of moving a higherair volume and/or may be more efficient in outputting anti-torque thrust44 by virtue of having a higher inertia. However, tail fan assembly 32may respond more quickly to yaw control inputs from the pilot or fromelsewhere, and be able to change speed rapidly for finer yaw control ofrotorcraft 10. The smaller diameter of tail fan assembly 32 also reducesthe tip speed, and therefore noise, of rotating fan blades 42. Thus,tail fan assembly 32 may be utilized when a reduced noise environment ispreferable, such as during air reconnaissance or clandestine operations.By utilizing tail fan assembly 32 for quicker and/or finer yawadjustments and tail fan assembly 30 to achieve higher maximumanti-torque thrust 44, anti-torque system 28 is able to utilize two ormore tail fans to achieve optimal responsiveness, maximum thrust andcost in managing the yaw of rotorcraft 10.

While motors 36, 40 are variable speed motors capable of having a largerange of RPM settings, in other embodiments motors 36, 40 may be fixedspeed or other types of motors. For example, either or both of motors36, 40 may be a stacked motor assembly in which two or more motors arestacked end-to-end and drive a single driveshaft to provide torque for arespective tail fan. Either or both of motors 36, 40 may be an electricmotor, hydraulic motor or mechanically-driven motor. Anti-torque system28 may utilize any number of motors to drive the tail fan assembliesincluded therein. For example, anti-torque system 28 may include asingle motor that drives both tail fan assemblies 30, 32. Tail fanassemblies 30, 32 are both shrouded, or fenestron, fantail or ducted,tail fan assemblies. While tail fan assemblies 30, 32 are both shroudedwithin vertical fin 34, in other embodiments tail fan assemblies 30, 32may be shrouded by ducts that are structurally independent from tailboom20 or vertical fin 34. Shrouding tail fan assemblies 30, 32 helps toreduce the edgewise flow on fan blades 38, 42, which can causeundesirable moments on tailboom 20. Although high inertia tail fanassembly 30 is located aft of low inertia tail fan assembly 32, in otherembodiments high inertia tail fan assembly 30 may be located forward oflow inertia tail fan assembly 32. In yet other embodiments, tail fanassemblies 30, 32 may be vertically, instead of horizontally, aligned.

Rotorcraft 10 includes flight control computer 48. In some embodiments,flight control computer 48 includes an anti-torque controller 50 thatcontrols the yaw of rotorcraft 10 using tail fan assemblies 30, 32.Anti-torque controller 50 may be in mechanical, electrical, wireless,computer or any other type of communication 52 with tail fan assemblies30, 32. Anti-torque controller 50 includes a yaw change determinationmodule 54 to determine an amount by which to change or correct the yawof rotorcraft 10. In determining the yaw adjustment for rotorcraft 10,yaw change determination module 54 may include and utilize a yaw ratesensor and/or a yaw position sensor. Anti-torque controller 50 alsoincludes a tail fan control module 56 to modify the yaw of rotorcraft 10using tail fan assemblies 30, 32. Tail fan control module 56 maydetermine the magnitude of anti-torque thrusts 44, 46 that are requiredto achieve the desired yaw of rotorcraft 10 as determined by yaw changedetermination module 54. Tail fan control module 56 may also determinehow quickly anti-torque thrust must be implemented so that the desiredyaw is achieved in a timely manner. Tail fan control module 56 may thusdetermine whether and how fast to rotate each tail fan assembly 30, 32,taking into account that tail fan assembly 30 has a higher rotationalinertia, and thus a lower angular acceleration, than tail fan assembly32. Anti-torque controller 50 thus enhances the yaw management ofrotorcraft 10 by selectively activating tail fan assemblies 30, 32depending on the thrust and responsiveness requirements of theoperational circumstance.

It should be appreciated that rotorcraft 10 is merely illustrative of avariety of aircraft that can implement the embodiments disclosed herein.Indeed, anti-torque system 28 may be implemented on any aircraft thatexperiences yaw movement. Other aircraft implementations can includehybrid aircraft, tiltrotor aircraft, tiltwing aircraft, quad tiltrotoraircraft, unmanned aircraft, gyrocopters, airplanes and the like.Anti-torque system 28 may also be utilized on rotorcraft having adistributed propulsion system with two or more rotors powered by anelectrical, hydraulic, mechanical or other energy source. As such, thoseskilled in the art will recognize that anti-torque system 28 can beintegrated into a variety of aircraft configurations. It should beappreciated that even though aircraft are particularly well-suited toimplement the embodiments of the present disclosure, non-aircraftvehicles and devices can also implement the embodiments.

Referring to FIGS. 2A-2B in the drawings, a variety of anti-torquesystem configurations are shown by which to differentiate the rotationalinertias of the tail fan assemblies therein. Referring to FIG. 2A,anti-torque system 100 includes low inertia tail fan assembly 102 andhigh inertia tail fan assembly 104 shrouded within vertical fin 106 oftailboom 108. High inertia tail fan assembly 104 has a higher rotationalinertia than low inertia tail fan assembly 102. Each tail fan assembly102, 104 includes a motor 110, 112 and fan blades 114, 116,respectively. While fan blades 114 and fan blades 116 each have the samelength, high inertia tail fan assembly 104 includes a larger number offan blades 116. Because high inertia tail fan assembly 104 includes alarger number of fan blades, fan blades 116 collectively have a higherrotational inertia, have a lower angular acceleration and are capable ofproducing a higher maximum anti-torque thrust than fan blades 114 of lowinertia tail fan assembly 102. Also contributing to the higherrotational inertia of high inertia tail fan assembly 104 iscircumferential tip ring 118 coupled to the outboard tips of fan blades116 and rotatable with tail fan assembly 104. Circumferential tip ring118 reduces gaps between fan blades 116 and a shroud or enclosure ring,in this case vertical fin 106, to reduce tip gap and efficiency lossesand increase thrust performance. Circumferential tip ring 118 may alsohelp to reduce the noise produced by high inertia tail fan assembly 104.

Referring to FIG. 2B, anti-torque system 120 includes low inertia tailfan assembly 122 and high inertia tail fan assembly 124 shrouded withinvertical fin 126 of tailboom 128. High inertia tail fan assembly 124 hasa higher rotational inertia than low inertia tail fan assembly 122. Eachtail fan assembly 122, 124 includes a motor 130, 132 and fan blades 134,136, respectively. Despite fan blades 134, 136 having the same length,high inertia tail fan assembly 124 achieves a higher rotational inertiathan low inertia tail fan assembly 122 by virtue of fan blades 136 beingformed from a different material than fan blades 134. In particular, fanblades 134 are formed from a lighter material than fan blades 136. Inone non-limiting example, fan blades 136 may be formed from an aluminumor aluminum alloy material and fan blades 134 may be formed from alighter composite or carbon-based material. Such composite orcarbon-based materials may be more expensive than the material fromwhich fan blades 136 are formed. If cost is a compelling designconstraint, composite, carbon-based or other expensive materials may bemore sparingly used throughout the two or more tail fan assemblies ofanti-torque system 120. In some embodiments, fan blades 134 may beformed from a low inertia and low strength material and fan blades 136may be formed from a high inertia and high strength material. Therotational inertias of tail fan assemblies 122, 124 may also bedifferentiated from one another by varying the widths of fan blades 134relative to fan blades 136. For example, width 138 of fan blades 134 maybe smaller than width 140 of fan blades 136. The widths 138, 140 of fanblades 134, 136 may differ whether or not fan blades 134, 136 are formedfrom the same material. In the illustrative embodiments, the fan bladesof the high inertia tail fan assemblies may have a higher collectivemass than the fan blades of the low inertia tail fan assemblies byvirtue of being longer, wider, more numerous, formed from a heaviermaterial or any other physical attribute.

Referring to FIGS. 3A-3B in the drawings, various anti-torque systemconfigurations for a rotorcraft are schematically illustrated. Referringto FIG. 3A, anti-torque system 200 includes four low inertia tail fanassemblies 202 a-202 d and two high inertia tail fan assemblies 204a-204 b each having respective fan blades rotated by a respective motor.Tail fan assemblies 202 a-202 d, 204 a-204 b are each shrouded byvertical fin 206 at the aft portion of tailboom 208. The overallconfiguration, position or footprint of tail fan assemblies 202 a-202 d,204 a-204 b may be tailored to fit within the confines of vertical fin206 or other aft portion of tailboom 208. While a particular number oflow inertia tail fan assemblies 202 a-202 d and high inertia tail fanassemblies 204 a-204 b are illustrated, anti-torque system 200 mayinclude any number of low or high inertia tail fan assemblies dependingon the requirements or desired attributes of the rotorcraft. Forexample, anti-torque system 200 may include the same or a differentnumber of low inertia tail fan assemblies relative to high inertia tailfan assemblies, and such tail fan assemblies may be positioned at anyforward or aft position along tailboom 208.

Referring to FIG. 3B, anti-torque system 210 includes three low inertiatail fan assemblies 212 a-212 c and three high inertia tail fanassemblies 214 a-214 c. Each tail fan assembly 212 a-212 c, 214 a-214 cincludes respective fan blades rotated by a respective motor. Lowinertia tail fan assembly 212 b and high inertia tail fan assembly 214 bare shrouded by vertical fin 216 at the aft portion of tailboom 218. Lowinertia tail fan assemblies 212 a, 212 c and high inertia tail fanassemblies 214 a, 214 c are open, or unshrouded, tail fan assemblies. Insome embodiments, unshrouded tail fan assemblies 212 a, 212 c, 214 a,214 c may be capable of changing pitch, or flapping, to counteractedgewise flow during flight, thereby helping to prevent undesirablemoments on tailboom 218. Anti-torque system 210 may include any ratio ofopen tail fan assemblies to shrouded tail fan assemblies. In onenon-limiting example, anti-torque system 210 may include three shroudedtail fan assemblies and three open tail fan assemblies, and such open orshrouded tail fan assemblies may be either high or low inertia dependingon the requirements and desired attributes of the rotorcraft. Forexample, all of the tail fan assemblies of anti-torque system 210 may beopen tail fan assemblies in some embodiments. In other embodiments, therotorcraft may exclude vertical fin 216 and the tail fan assemblies ofanti-torque system 210 may be either open or include ducts that arestructurally separate, yet coupled, to tailboom 218.

The flight control computers of the present embodiments preferablyinclude computing elements such as non-transitory computer readablestorage media that include computer instructions executable byprocessors for controlling flight operations. The computing elements maybe implemented as one or more general-purpose computers, special purposecomputers or other machines with memory and processing capability. Thecomputing elements may include one or more memory storage modulesincluding, but is not limited to, internal storage memory such as randomaccess memory, non-volatile memory such as read only memory, removablememory such as magnetic storage memory, optical storage, solid-statestorage memory or other suitable memory storage entity. The computingelements may be implemented as microprocessor-based systems operable toexecute program code in the form of machine-executable instructions. Thecomputing elements may be selectively connectable to other computersystems via a proprietary encrypted network, a public encrypted network,the Internet or other suitable communication network that may includeboth wired and wireless connections.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. An anti-torque system for a rotorcraftcomprising: a first tail fan assembly including a plurality of first fanblades; and a second tail fan assembly including a plurality of secondfan blades; wherein, the first tail fan assembly has a larger diameterthan the second tail fan assembly; and wherein, the first fan bladeshave a larger rotational inertia than the second fan blades such thatthe second fan blades experience a larger angular acceleration than thefirst fan blades in response to torque, thereby providing yaw controlfor the rotorcraft.
 2. The anti-torque system as recited in claim 1wherein the first and second fan assemblies further comprise fixedpitch, variable rotational speed fan assemblies.
 3. The anti-torquesystem as recited in claim 1 wherein the first and second fan assembliesfurther comprise variable pitch, fixed rotational speed fan assemblies.4. The anti-torque system as recited in claim 1 wherein the first fanblades and the second fan blades are formed from the same material. 5.The anti-torque system as recited in claim 1 wherein the first fanblades are longer than the second fan blades.
 6. The anti-torque systemas recited in claim 1 wherein the first fan blades are formed from adifferent material than the second fan blades.
 7. The anti-torque systemas recited in claim 1 wherein the plurality of first fan blades includea larger number of fan blades than the plurality of second fan blades.8. The anti-torque system as recited in claim 1 wherein the first fanblades are wider than the second fan blades.
 9. The anti-torque systemas recited in claim 1 wherein the first fan blades further comprise acircumferential tip ring.
 10. The anti-torque system as recited in claim1 wherein at least one of the first or second tail fan assembliesfurther comprise a shrouded tail fan assembly.
 11. The anti-torquesystem as recited in claim 1 wherein at least one of the first or secondtail fan assemblies further comprise an open tail fan assembly.
 12. Theanti-torque system as recited in claim 1 wherein the first tail fanassembly further comprises a plurality of first tail fan assemblies andwherein the second tail fan assembly further comprises a plurality ofsecond tail fan assemblies, the plurality of first tail fan assemblieshaving larger diameters than the plurality of second tail fanassemblies.
 13. The anti-torque system as recited in claim 1 furthercomprising at least one motor configured to provide torque to the firstand second fan blades.
 14. The anti-torque system as recited in claim 13wherein the at least one motor further comprises first and secondmotors, the first tail fan assembly including the first motor, thesecond tail fan assembly including the second motor.
 15. The anti-torquesystem as recited in claim 13 wherein the at least one motor furthercomprises an electric motor.
 16. The anti-torque system as recited inclaim 13 wherein the at least one motor further comprises a variablespeed motor.
 17. A rotorcraft comprising: a fuselage; a tailboomextending from the fuselage, the tailboom having an aft portion; and ananti-torque system at least partially located at the aft portion of thetailboom, the anti-torque system further comprising: a first tail fanassembly including a plurality of first fan blades; and a second tailfan assembly including a plurality of second fan blades; wherein, thefirst tail fan assembly has a larger diameter than the second tail fanassembly; and wherein, the first fan blades have a larger rotationalinertia than the second fan blades such that the second fan bladesexperience a larger angular acceleration than the first fan blades inresponse to torque, thereby providing yaw control for the rotorcraft.18. The rotorcraft as recited in claim 17 wherein the aft portion of thetailboom further comprises a vertical fin, and wherein the first andsecond tail fan assemblies are coupled to the vertical fin.
 19. Therotorcraft as recited in claim 17 further comprising a flight controlcomputer including an anti-torque controller in communication with thefirst and second tail fan assemblies, the anti-torque controlleroperable to control the yaw of the rotorcraft using the first and secondtail fan assemblies.
 20. The rotorcraft as recited in claim 19 whereinthe anti-torque controller further comprises a yaw change determinationmodule operable to determine a yaw adjustment for the rotorcraft and atail fan control module operable to modify the yaw of the rotorcraftusing the first and second tail fan assemblies.